Process for coating uranium alloy members



Unite tates The present invention relates to a process for producing metallic coatings on uranium members, more particularly to the process for producing high adhesion chromium coatings on uranium and alloys thereof by chemical displacement.

A conventional fuel element for nuclear reactors comprises a solid fuel member of uranium or of alloys thereof, enclosed in an aluminum casing. It is desirable that a thin metallic intermediate layer, such as of nickel, be interposed between the uranium and the aluminum casing. One reason for the intermediate nickel layer is to prevent an undesired reaction of aluminum with uranium which would form an intermetallic compound UAl This reaction would result in the destruction of the aluminum casing during the operation of the reactor within a relatively short period of time. In addition, the intermediate nickel layer will provide through diffusion bonding, an intimate contact between the uranium and the aluminum. This intimate contact also results in providing excellent heat transfer properties between the aluminum casing and the fuel therein.

However, it has been found that various metals, such as nickel, cannot be deposited upon uranium and its alloys by electroplating without producing a film of foreing matter, composed presumably of an oxide, between the uranium and the metal to be deposited. This film is disadvantageous since it prevents either wholly or par tially an adhesion by atomic forces and a subsequent diffusion bonding between the uranium and the metal deposited thereon.

This film of foreign matter which is not initially visible, is formed primarily in water and/ or oxygen bearing media. However, this film of foreign matter can be recognized by its black color which occurs after heating the film to higher temperatures for a short period of time. The same phenomenon of the film formation will also occur if the uranium is left at room temperature for a longer period of time. This black colored film will also be formed even under very thick galvanic deposits which may surround the uranium completely and which prevents the passage of any oxygen whatsoever from the atmosphere to the uranium.

The previously proposed method of solving the above problem involved roughening the surface of the uranium fuel member in order to obtain at least a mechanical adhesion. This roughening of the surface was either produced by blasting with steel particles or by special chen ical pickling processes. In the case of nickel deposited upon a roughened uranium surface there is obtained only a limited adhesion and the diffusion resulting from the formation of intermetallic compounds under increased pressure and elevated temperatures either does not occur at all, or if it occurs, occurs only in spots and then to a limited extent. As a result, such fuel elements were characterized by having a low adhesion between the aluminum casing and the uranium and could only withstand very small thermal and mechanical loads without the aluminum casing becoming loosened from the fuel therein.

It is, therefore, the principal object of the present invention to provide a novel and improved process for applying aten a thin, highly adhesive metallic layer onto uranium and alloys thereof.

It is another object of the present invention to provide a process for forming a fuel element comprising a metal from the actinide group or alloys thereof for a nuclear reactor wherein the outer metallic casing is closely bonded to the fuel member therein.

The present invention essentially comprises immersing a member formed of a metal from the actinide group or alloys thereof into an aqueous solution containing chromium ions so that a metallic chromium coating is formed upon the metallic member by chemical displacement or by electrodeposition. When uranium and alloys thereof are immersed into aqueous solutions of CrO to which has been added soluble materials containing chloride and/ or fiuoride ions as well as small quantities of soluble materials containing sulfate ions, the member will be coated with a firmly adherent chromium film which is light to dark gray in appearance. The quality of this film and the rate of deposition of the film are functions of the concentration of the aforementioned substances in the aqueous solution, the temperature of the bath, the nature of the uranium alloy, and the microstructure of the same. Such chromium coatings or films can be successfully formed on uranium and its alloys from solutions within the following range of concentrations.

pH value 1 Temperature C 2040 Time minutes 1-10 Bearing in mind the above figures, the optimum composition of a dipping bath for a particular uranium alloy can be readily determined.

After the chromium coated member has been thoroughly rinsed in water, a variety of metals suitable for electroplating such as nickel can be electrodeposited on the member so as to :firmly adhere to the chromium film. By means of this intermediate layer various metals for encasing the fuel member, such as aluminum, can be firmly bonded to the uranium by means of a diffusion process. As a result, a desired diffusion bond between the uranium and outer layer such as nickel is obtained which has great uniformity. This bond will withstand substantially higher thermal and mechanical loads than any other types of bonds previously produced to date.

The resulting chromium film is extremely thin and difcult to detect under the microscope. At optimum bath composition a dense chromium film is formed on the surface of the uranium alloy. After completely covering the surface, the film does not become any thicker independent of the time of the dipping treatment. The maximum thickness observed under the microscope was less than 0.1 (0.004 mil).

When the above designated CrO solution was used, it was found that during the coating of uranium with chromium a portion of the Cr+ ions was reduced to Cr+ ions at the same time. A significant proportion of Cr+ ions in the solution may render the dipping bath unservicable. The formation of such detrimental Cr+ ions can be eliminated to a large extent if metallic lead or lead alloys in sheet or rod form are inserted into the dipping bath. As a result, the reoxidation of Cr+ ions to Cr ions will occur at the lead surface similar to the action in chromium electroplating baths.

This same result can also be achieved by connecting the dipping bath to a low voltage (1-10 volts), low amperage (0.1-1 amp/dm?) DC. or A.C. electrical source by means of electrodes. In this process it is of minor importance whether or not the uranium member which is to be coated is connected to the electrical power circuit during the actual dipping operation.

In order to describe in detail a specific embodiment of the following invention, the following operating example is set forth solely as being illustrative of the process of the present invention, and is not to be considered as limiting this process in any way.

Example 1 A typical nuclear fuel member of the uranium alloymolybdenum type consisting of U3.5 Mo0.1 Al alloy was finished to the prescribed dimensions. This uranium alloy member was degressed in a conventional manner and pickled in semi-concentrated nitric acid for about minutes. The uranium alloy members were then cleaned anodically in a solution consisting of 2 parts H PO and 1 part methyl alcohol. After this preparatory treatment, the uranium alloy members were dipped for 1 to 3 minutes into an aqueous solution consisting of 70 grams per liter CrO 2.5 grams per liter NaF, 2.5 grams per liter NaCl, 5 cu. cc. per liter H SO having a pH value of less than 1.0 and a temperature of 30 C. After the previously designated period of time has elapsed, the uranium alloy members were coated with a firmly adherent chromium film which was gray in color. The uranium members were then thoroughly rinsed in water and were then ready for nickel plating by galvanic means in a conventional and well known manner.

Based on similar electrochemical potentials, other actinides in addition to uranium, including thorium, plutonium and their respective alloys, can be coated with a chromium film in a like manner. The specific compositions of the bath and conditions of the dipping will vary somewhat depending on the exact composition of the :member being coated.

Besides the manufacture of nucelar fuel elements as described herein, the coating of uranium and alloys thereof by closely adhering metallic layers is important in many other applications, including affording protection against corrosion and wear in radiation shielding blocks and counterweights in aircraft, both of which are fabricated from depleted metallic uranium and alloys thereof.

Thus it can be seen that the present invention provides a novel and improved process for applying a chromium film to a fuel member comprising a metal from the actinide group or an alloy thereof.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What is claimed as this invention is:

1. A process for applying a firmly adherent chromium intermediate layer to the fuel of a fuel element for a nuclear reactor wherein the fuel comprises a U-3.5 Mo0.1 Al alloy member, the step of said process comprising cleaning said uranium alloy member, and [immersing the cleaned uranium alloy member for approximately 1 to 3 minutes into an aqueous solution at a temperature of approximately 20 C.30 C. and a pH value 1, said solution comprising approximately grams per liter of chromium oxide CrO approximately 2.5 grams per liter NaF, approximately 2.5 grams per liter NaCl, approximately 5 cubic centimeters per liter H SO so that a metallic chromium layer is formed on the alloy member by a chemical displacement.

2. A process for applying a firmly adherent chromium intermediate layer to the fuel of a fuel element for a nuclear reactor, said fuel comprising an aetinide metal from the group consisting of uranium, thorium, plutonium and alloys thereof, the steps of said process comprising cleaning said aetinide metal and immersing the cleaned aetinide metal for approximately 1 to 10 minutes in an aqueous solution at a temperature of approximately 20 C. to 40 C. and apH value less than 1, said solution comprising approximately 20 to 200 grams per liter CrO approximately 1 to 100 grams per liter soluble halogen compounds from the group consisting of HCl, HF, NaCl, NaF, HClO HC1O NaClO H'BF and mixtures thereof, and approximately 0.5 to 20 grams per liter soluble sulfate compounds from the group consisting of H and salts thereof whereby said chromium intermediate layer is formed on said aetinide metal by a chemical displacement.

3. The process of claim 2, wherein said aetinide metal is uranium.

4. The process of claim 2, wherein said aetinide metal is the uranium alloy U3.5 Mo0.1 Al.

5. The process of claim 2, wherein the formation of Cr+ ions is prevented by the addition to said solution of a material from the group consisting of lead and alloys thereof.

6. A process for applying a firmly adherent chromium intermediate layer to the fuel of a fuel element for a nuclear reactor, said fuel comprising U3.5 Mo0.1 Al alloy member, the steps of said process comprising cleaning said alloy member, and immersing the cleaned alloy member for approximatley 1 to 3 minutes in an aqueous solution at a temperature of approximately 20 C. to 30 C. and a pH value less than 1, said solution consisting essentially of approximately 70 grams per liter CrO approximately 2.5 grams per liter NaCl and approximately 5 cubic centimeters per liter H 80 whereby said chromium intermediate layer is formed on said alloy member by a chemical displacement.

References Cited by the Examiner UNITED STATES PATENTS 2,114,151 4/1938 Romig 1486.2 2,127,202 8/1938 Boyle 148 6.2 2,507,956 5/1950 Bruno et a1. 1486.2 2,851,766 9/1958 Gray 204-15 X 2,894,884 7/1959 Gray 2041.5

ALFRED L. LEAVITT, Primary Examiner.

REUBEN EPSTEIN, RICHARD D. NEVIUS, R. S.

KENDALL, Assistant Examiners. 

1. A PROCESS FOR APPLYING A FIRMLY ADHERENT CHROMOUM INTERMEDIATE LAYER TO THE FUEL OF A FUEL ELEMENT FOR A NUCLEAR REACTOR WHEREIN THE FUEL COMPRISING A U-3.5 MO-0.1 AL ALLOY MEMBER, THE STEP OF SAID PROCESS COMPRISING CLEANING SAID URANIUM ALLOY MEMBER, AND IMMERSING THE CLEANED URANIUM ALLOY MEMBER FOR APPROXIMATELY 1 TO 3 MINUTES INTO AN AQUEOUS SOLUTION AT A TEMPERATURE OF APPROXIMATELY 20*C.-30*C. AND A PH VALUE <1, SAID SOLUTION COMPRISING APPROXIMATELY 70 GRAMS PER LITER OF CHROMIUM OXIDE CRO3, APPROXIMATELY 2.5 GRAMS PER LITER NAF, APPROXIAMATELY 2.5 GRAMS PER LITER NACL, APPROXIMATELY 5 CUBIC CENTIMETERS PER LITER H2SO4, SO THAT A METALLIC CHROMIUM LAYER IS FORMED ON THE ALLOY MEMBER BY A CHEMICAL DISPLACEMENT. 